In many automated production environments moving conveyors are used to transport components and/or production parts. Many applications require that the load carried by the conveyor be affirmatively held in place on the conveyor in order to facilitate one or more particular steps in the production or fabrication process. Often, it is also necessary to precisely position the load on the conveyor to facilitate a precision cutting, measuring or assembly step. To facilitate such positioning, conveyors are traditionally outfitted with one or more outwardly extending lugs which are adapted to engage particular loads carried by the conveyor. One common approach is to use a pair of spaced apart parallel lugs that have precisely aligned front surfaces.
A good example of an assembly that incorporates a conveyor system of the type described above is a fingerjoint cutting machine used within the lumber industry to make the precision cuts necessary to form finger joints. Specifically, the conveyor includes two or more parallel chains that form an endless loop and a multiplicity of load stations are disposed about the endless loop at relatively equidistant intervals. The conveyor may be operated at a wide variety of speed and by way of example, speeds in the vicinity of 100 feet per minute are common. Each load station includes two or more lugs which are precisely positioned adjacent each other to carry a single piece of wood stock. The lugs are steel or aluminum blocks mounted to the moving chains. By way of example, a representative conveyor may have on the order of 40-100 load stations with the lugs placed on six inch centers. Air bags are used to push down upon the finger-joint stock to firmly hold it in place as the conveyor carries the stock through various cutting and gluing stations.
If for any reason the cuts are not precisely made, the resultant finger-joints are defective due to openness in the joint area. Within conventional finger-joint production lines, the majority of the open joint defects that actually occur are directly attributable to misalignment of the lugs within one or more particular load stations. The precise tolerances required for the finger-joints dictate that if the lugs are out of alignment by as little as 2-5 mils, the cuts made by the finger-joint machine will produce unacceptable parts having open joints. These strict tolerances have created enormous difficulties within the industry because even if the lugs in only one load station are out of alignment, a large number of unacceptable products are produced and there is no easy way to identify the source of the defective parts. Therefore, the defective parts are often not discovered until well downstream of the cutting machines at which time the part is typically a complete loss or requires expensive rework to salvage. When defects begin occurring, the plant operators are faced with the decision of having to shut down the entire production line while the alignment of the lugs in each of the alignment stations are checked, (a process that may take 45 minutes) or to accept the high defect rate that will necessarily occur due to the lug misalignment. Neither option is an acceptable scenario.
To minimize the occurrence of lug misalignment, the alignment of each lug pair is typically checked by hand after every shift and aligned by hand with a square. Such a process is time consuming, expensive, and does not eliminate the development of misalignment during a production shift. It will be appreciated that the above described misalignment problem can cause difficulties in a wide variety of production and materials handling operations outside of the finger-joint industry, although the severity of the consequences of slight misalignments may vary a great deal from operation to operation. Other applications require even greater precision then that described above. For example, many metalworking applications require alignment resolution on the order of 1 mil or less. Therefore, there is a need for a device capable of accurately testing the alignment of devices such as conveyor lugs on the fly. One solution was proposed by Nowakowski in co-pending application Ser. No. 228,548 , filed Aug. 4, 1988, the present invention is an improvement of that solution.